Grinding method and coolant therefor

ABSTRACT

The method of grinding workpieces of vitreous materials having anionically charged surfaces when ground including the application of a cationic coolant to the workpieces for attraction to surfaces thereof being ground and lubrication of the effective grinding surfaces of tools used in the grinding operation. The cationic coolant comprises a solution of polyethylenimine in a solvent which minimizes friction between the tool and workpiece components with a reduction of heating thereof and enhancement of grinding results.

Unlted States Patent 1 [111 3,922,821

Canterbury Dec. 2, 1975 [5 1 GRINDING METHOD AND COOLANT 3.251.778 5/1966 Dickson 252/392 THEREFOR 3.262.791 7/1966 Dickson 252/31'3 3.747.284 7/1973 Lyczko... 51/322 X i Inventor: Bradford Canterbury, sturbridgc, 3.823.515 7/1974 Coes 51/322 Mass.

[73] Assignee: American Optical Corporation, Primary Kelly gomhbricigev Mass Attorney, Agent. or FirmH. R. Berkenstock. Jr.; William C. Nealon 22 F1led: Apr. 18, 1975 [21] Appl. No.: 569,245 57 ABSTRACT Related Applicatio Data The method of grinding workpieces of vitreous materi- [62] Division of Ser. No. 397,480, Sept. 14, I973, als having anionically charged surfaces when ground abandoned including the application of a cationic coolant to the workpieces for attraction to surfaces thereof being i521 U-S. Cl. 51/284 R; 51/322 ground and lubrication of the effective grinding sur- [51] Int. Cl.2 B248 H00 faces of tools used in the grinding operation. The cati- Field Of Search 3 R. 8 onic coolant comprises a solution of polyethylenimine 252/3 2; 26 /2 EN in a solvent which minimizes friction between the tool and workpiece components with a reduction of heat- 1 References Cited ing thereof and enhancement of grinding results.

UNITED STATES PATENTS 12/1939 Ulrich 260/2 EN 9 Claims, 4 Drawing Figures U.S Pltfillt Dec. 2, 1975 3,922,821

N humm- GRINDING METHOD AND COOLANT THEREFOR This is a division, of application Ser. No. 397,480 filed Sept. 14, 1973, and now abandoned.

Field of the Invention This invention relates to the grinding of vitreous materials with particular reference to grinding operations using liquid coolants for enhancing the grinding action.

BACKGROUND OF THE INVENTION Surface and edge grinding operations performed upon glass and ceramic workpieces have heretofore been attended with problems of rapid dulling of the grinding tools, long grinding cycles and unduly rough or excessively scratched final surfaces, all of which have resulted in large scrap yields, high manufacturing costs and less than optimum end product results.

Coolants used heretofore in efforts to enhance grinding actions and output have fallen into the general categories of: (1) Straight oils usually light mineral oils or kerosene; (2) Water-soluble emulsions which contain oil and surfactants for emulsifying the oil; (3) Semisynthetic types which contain relatively small amounts of oil and large percentages of surfactants or detergents; and (4) Synthetic, chemical or solution types which contain no oil but rely upon various chemical compounds to achieve desired properties.

Coolant formulations in the categories of items (1), (2), and (3) which require surfactants traditionally use anionic or non-ionic surface active agents for reducing surface tension, supplying lubricity and emulsifying oil content.

Cationic fluids found in the general class of item (4) have, heretofore, not performed well as glass or ceramic grinding coolants nor have they been readily accepted for such purposes in view of one or more of their drawbacks including: excessive foaming, offensive odor, the depositing of difficult to remove residue upon machines and work, the slow settling of grinding residue therein and its not settling in a hard or otherwise easy to remove form. Additionally, the lubricating properties of prior art cationic fluids are less than optimum and some have been excessively corrosive to machinery.

The present invention overcomes the aforementioned drawbacks and corollary problems in prior art glass grinding operations through the provision of a fluid coolant which is cationic in nature and readily attracted to negatively charged cut or ground surfaces of glass or ceramic materials. This coolant is free flowing and highly lubricative for keeping glass cutting tools sharp and free cutting with minimum pressure. It is further substantially odorless and non-foaming in general u and promotes rapid settling of fines or grinding residue with hard packing thereof for ready removal with minimal loss or waste of the coolant.

SUMMARY OF THE INVENTION:

The objectives of the present invention are accomplished by the use of a grinding coolant comprised of polyethylenimine in a solvent of, for example, water or ethylene glycol. This coolant is applied continuously to the surfaces of the workpieces and the effective grinding portions of the grinding tools.

Due to the cationic nature and lubricating properties of polyethylenimine, the present coolant becomes attracted to the inherently negatively charged surfaces of vitreous workpieces being ground and lubricates the workpiece-tool interface, minimizing tool wear and heating due to friction. Additionally, in view of the free-flowing characteristics of the subject coolant, it readily removes grinding swarf and permits rapid settling of the swarf in the used coolant as a relatively hard, easily removable residue.

According to the invention, grinding tools are prevented from loading or glazing and are kept free cutting throughout normal grinding cycles. Minimal tool-workpiece interfacial pressures are required and grinding cycle times are substantially shortened with the realization of considerable economical advantage and improved product quality.

Other objects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawmgs.

DESCRIPTION OF THE DRAWINGS FIGS. 1, 2, and 3 are diagrammatic illustrations of respectively different exemplary types of grinding operations which may be performed according to the invention; and

FIG. 4 is a greatly enlarged fragmentary crosssectional view of portions of a workpiece and grinding tool wherewith principles of the invention are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Grinding operations with which the present invention is particularly concerned involve the grinding of surfaces and/or edges of vitreous workpieces such as, for example, glass lenses and lens blanks, window pane or sheet glass or surfaces and edges of blown, pressed or otherwise formed hollow containers of glass or ceramic materials and solid glass or ceramic articles and the like.

While the invention will be described hereinafter as relating more particularly to the operation of glass or ceramic grinding, it should be understood that other operations such as drilling, trepaning, or sawing (not shown) may be performed according to the invention and with the benefit thereof.

Referring more particularly to FIGS. 1, 2, and 3, these illustrations exemplify three types of grinding operations normally performed upon glass lenses or lens blanks l0, l0, and 10", it being understood that other glass articles such as panes or sheets of glass or other vitreous materials may replace any one or another of the illustrated lens blanks.

In FIG. 1 there is depicted a glass grinding operation commonly referred to in the ophthalmic industry as a generating operation. A lens blank 10 is secured to an axial rotatable holder 12 and a cup-shaped tool 14 is brought into grinding relationship with a surface 16 of blank 10 which is to be ground, e.g. to a curved surface shape.

Tool 14 having an annular effective abrating surface 18, preferably formed of metal bonded diamond particles, is pressed against surface 16. A pressure of approximately lOO lbs. per square inch may, for example, be used. Tool 14 and work holder 12 are, in this case, both rotated about their respective axes simultaneously and preferable in opposite directions to effect a grinding action extending over the entire surface 16.

During the grinding of lens blank 10, liquid coolant 20 is continuously applied to the tool 14 and surface 16 through a circulating system diagrammatically illustrated as comprising the emitting nozzle 22. This coolant 20, after use between tool 14 and surface 16 flows off lens blank and is collected in a settling tank or similar apparatus (not shown) and recirculated to nozzle 22 for reuse. The settling tank in its simplest form may be a deep pan or similar container into one end of which the used coolant is collected and from an opposite end of which this coolant is recirculated. Swarf or grinding residue carried by the coolant settles in the bottom of the container for removal therefrom and disposal. Coolant settling tanks and circulating mecha nisms being well known and common in the art will not be shown or described in detail herein. Those wishing greater detail of the mechanics of such systems may refer to US. Pat. No. 2,724,218 wherein an exemplary system is shown and described.

The grinding operation illustrated in FIG. 2 is of the type commonly known and referred to in the art of grinding lenses as a milling operation. In this case the lens blank 10 is secured to a stationary support 12' while tool 14, being generally similar to tool 14 of FIG. 1, is caused to rotate about its axis and simultaneously move arcuately, i.e. in the direction of arrow 24, across the surface 16' of the lens blank at a depth of cut equal to that desired to be produced upon the lens blank 10'. In this operation, coolant 20 supplied by nozzle 22 is continuously applied to the surface 16 and the grinding surface 18 of tool 14'.

The third exemplary grinding operation illustrated in FIG. 3 is commonly known in the art of lens grinding and other arts as an edging operation. Here, an edge of lens blank 10 is moved into engagement with a rotatable grinding disc or wheel 2 having a grinding surface 28. The lens blank 10' may be rotated relative to wheel 26 for completion of edge grinding thereof about its periphery. Alternatively, in the case of a flat glass plate or the like replacing lens blank 10 as the workpiece, such a plate may be merely moved linearly across the grinding surface 28 of wheel 26, It is to be understood that the grinding surface 28 may be grooved as illustrated or flat for effacing beveled 0r flat edge grinding operations respectively. In all cases, however, liquid coolant 20 supplied by nozzle 22" is caused to flow against and over the grinding surface 28 of wheel 26.

In all of the grinding operations illustrated in FIGS. 1, 2, and 3, as well as others mentioned hereinabove and not illustrated, the liquid grinding coolant 20 which is used according to the present invention comprises a solution of polyethylenimine in a solvent of water of ethylene glycol.

Polyethylenimine of a pre-selected molecular weight within a range of from approximately 600 to 100,000 in the proportion of from approximately 0.1% to 5.0% in the selected solvent has been found to produce the improved results outlined hereinabove of preventing grinding tools such as tools 14, 14' or 26 from loading or glazing and keeping the tools free cutting throughout normal grinding cycles with the accomplishment of shortened grinding cycles and improved end product result.

The liquid coolant, being cationic in nature, becomes readily attracted to a negatively charged surface 39 (FIG. 4) of a vitreous workpiece, e.g. article A being ground by a tool T of the diamond impregnated type,

4 for example. The cationic or positively charged coolant 20, being attracted to the negatively charged surface 30, forms a lubricating interface between surface 30 and tool T, keeping the 'tool clean and free cutting and preventing overheating by excess friction.

FIG. 4 is intended to illustrate with article A and tool T, a typical workpiece-tool relationship encountered in the art of generating, milling or edging vitreous workpieces with tools such as 10 or 10 (FIGS. 1 and 2, for example). This, however, is not intended to exclude other grinding or cutting operations such as edging (FIG. 3) sawing, drilling or trepaning which may benefit from the application of coolant 20 as it is described herein.

Exemplary coolant 20 formulations are:

EXAMPLE I 170 milliliters polyethylenimine having a molecular weight of approximately 1800, a wt% (as C I-I N) of approximately 99, a density in lbs/gal at 25C of approximately 8.67, a viscosity (Brookfield at 25C) of approximately 8,50015,000 cps and a pH of approximately 10.6 when in a 5% aqueous solution; the polyethylenimine being in solution with approximately 16,830 milliliters water.

EXAMPLE II 16485 milliliters water and 515 milliliters polyethylenimine of a molecular weight between 40,000 and 60,000 and having a wt% as (C I-I N) of approximately 33, a density in lbs/gal at 25C of approximately 8.79, a viscosity (Brookfield at 25C) of less than 5,000 cps and a pH of approximately 10.0 when in a 5% aqueous solution.

The foregoing Examples I and II represent solutions containing approximately one percent polyethylenimine and produce the following results under tests designed to simulate severe grinding operations according to the following parameters:

Test Glass White crown glass in blanks weighing between 158 and 160 grams having a diameter of approximately 57.0 mm with approximately 2.7 cubic inches removed from one surface by grinding during each test.

Tool

Metal bonded diamond grinding surface with a diamond size between 270 and 325 mesh in a concentration of approximately 50. Outer diameter of tool being 1.50 inches and inner diameter being 1.25 inches.

Pressure Pressure between tool and workpiece being approximately 200 lbs. per square inch with a contact area of approximately one square inch.

Speeds Speed of rotation of the tool being approximately 1,100 rpm and speed of rotation of glass workpiece approximately 700 rpm in a direction opposite to the rotation of the tool.

Coolant Flow Flow rate of coolant being approximately 2,500 milliliters per minute.

Duration of Test Each test run being approximately 1 /2 hours. Results of Tests: 1

TABLE 1 Glass Removal Rate Total Glass (cubic inches per min.) Removed Ave. Ave. Ave. (cubic 1-30 31-60 61-90 inches) min. min. min. 1% Hr.

Example I Polyethylenimine .234 .192 .184 18.300

Example ll Polyethylenime .293 .267 .247 24.177

ln grinding operations conducted under identical conditions to those described hereinabove but with a prior art coolant of a solution of Ethylene Glycol, in water, the glass removal rate averaged .070 cubic inches during the last half hour of the l /2 hour test and maximum total glass removed was less than 11 cubic inches.

This test, when conducted with the coolants of Examples l and 11 showed average glass removal rates of 0.184 and 0.247 respectively during the last half hour of the l /2 hour test with total glass removed being greater than 18 cubic inches and as high as 24 cubic inches.

From the foregoing description of the invention and tabling of results of tests conducted according thereto, it can be seen that the grinding coolant of the present invention, which involves a new use for polyethylenimine, functions to keep grinding tools free of loading and glazing and continuously free cutting for shortening grinding cycle time and/or greatly increasing glass removal rates. Additional beneficial factors are rapid settling of swarf or grinding residue and the settling of such residue into a relatively solid and hard form which may be conveniently and easily removed from the used coolant for disposal without appreciable waste or loss of coolant. The grinding residue settles rapidly into a relatively hard and dry mass easily removable from the coolant. The present coolant obviates foaming problems and remains essentially indefinitely transparent whereby under certain circumstances requiring surveillance of the cutting action, such action may be visually monitored through the coolant. Further in view of the current relatively low cost of polyethylenimine and its adaptability to reuse over indefinite periods of time under normal working conditions, a considerable economical advantage in coolant cost and reduction in machinery set-up time may be realized. Rust or corrosion inhibitors and/or buffers to retard rise of pH, as well as germicides in some cases of need, may be added to the coolant of the present invention according to the usual practice in the art of grinding with liquid coolants. Al-

kali metal borates and alkali metal nitrites are well known rust inhibitors for steel. Benzotriazole is conventionally used to inhibit the corrosion of copper and other nonferrous metals.

1 claim:

1. The method of grinding a workpiece of vitreous material with a grinding tool comprising the steps of:

bringing the workpiece and tool into grinding relationship with each other;

producing relative movement between said workpiece and tool to introduce a grinding action upon said workpiece at points of contact between said tool and said workpiece along a surface thereof to be ground, said surface being anionically charged when ground; and

applying to said surface of said workpiece a coolant comprising the mixture of a large proportion of a free flowing liquid in which polyethylenimine is soluble and a relatively small proportion of polyethylenimine rendering the mixture cationic and lubricative; and workpiece and heating thereof is minimized with enhancement of the grinding action and its result.

2. The method according to claim 1 wherein said liquid is water, said polyethylenimine is of a preselected molecular weight within the range of from approximately 600 to 100,000 and is in the proportion of from approximately 0.1% to 5.0% in said coolant.

3. The method according to claim 2 wherein said polyethylenimine is in the proportion of approximately 1% in said coolant.

4. The method according to claim 3 wherein said polyethylenimine is of a preselected molecular weight of approximately 1,800.

5. The method according to claim 3 wherein said polyethylenimine is of a preselected molecular weight of from approximately 40,000 to 60,000.

6. The method according to claim 1 wherein said liquid is ethylene glycol, said polyethylenimine is of a preselected molecular weight within the range of from approximately 600 to 100,000 and is in the proportion of from approximately 0.1 to 5.0% in said coolant.

7. The method according to claim 6 wherein said polyethylenimine is in the proportion of approximately 1% in said coolant.

8. The method according to claim 7 wherein said polyethylenimine is of a preselected molecular weight of approximately 1,800.

9. The method according to claim 7 wherein said polyethylenimine is of a preselected molecular weight of from approximately 40,000 to 60,000.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO: 3,922,821

DATED 1 December 2, 1975 |NVENTOR(S) Bradford Canterbury It is certified that error appears in the ab0ve-identified patent and that said Letters Patent are hereby corrected as shown below.

In claim 1, line 21, after "cative; insert --whereby friction between said tool-- Signed and Sealed this twentieth Day of April1976 {SEAL} A trest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (mmnissinner of Parents and Trademarks 

1. The method of grinding a workpiece of vitreous material with a grinding tool comprising the steps of: bringing the workpiece and tool into grinding relationship with each other; producing relative movement between said workpiece and tool to introduce a grinding action upon said workpiece at points of contact between said tool and said workpiece along a surface thereof to be ground, said surface being anionically charged when ground; and applying to said surface of said workpiece a coolant comprising the mixture of a large proportion of a free flowing liquid in which polyethylenimine is soluble and a relatively small proportion of polyethylenimine rendering the mixture cationic and lubricative; and workpiece and heating thereof is minimized with enhancement of the grinding action and its result.
 2. The method according to claim 1 wherein said liquid is water, said polyethylenimine is of a preselected molecular weight within the range of from approximately 600 to 100,000 and is in the proportion of from approximately 0.1% to 5.0% in said coolant.
 3. The method according to claim 2 wherein said polyethylenimine is in the proportion of approximately 1% in said coolant.
 4. The method according to claim 3 wherein said polyethylenimine is of a preselected molecular weight of approximately 1,800.
 5. The method according to claim 3 wherein said polyethylenimine is of a preselected molecular weight of from approximately 40,000 to 60,000.
 6. The method according to claim 1 wherein said liquid is ethylene glycol, said polyethylenimine is of a preselected molecular weight within the range of from approximately 600 to 100,000 and is in the proportion of from approximately 0.1 to 5.0% in said coolant.
 7. The method according to claim 6 wherein said polyethylenimine is in the proportion of approximately 1% in said coolant.
 8. The method according to claim 7 wherein said polyethylenimine is of a preselected molecular weight of approximately 1,800.
 9. The method according to claim 7 wherein said polyethylenimine is of a preselected molecular weight of from approximately 40,000 to 60,000. 